1. Field of the Invention
This invention relates generally to an end block for a battery and, more particularly, to an end block with a composite structure having a low density, high strength core enveloped by a light weight inert material.
2. Description of the Prior Art
Electrochemical cells utilizing bipolar cell designs having reactive electrodes are well known. Conventional flowing electrolyte bipolar batteries are typically comprised of "stack" of cells, an electrolyte pump, an electrolyte reservoir, a cooling element, and external studs in electrical communication with the terminal electrodes. Each cell is comprised of an electrode upon which the anodic reaction takes place and an electrode upon which the cathodic reaction takes place.
In a typical bipolar battery, each electrode comprises two "poles", such that the anodic reaction occurs on one side of the electrode and the cathodic reaction occurs on the opposite side of the same electrode. Thus, in contrast to a monopolar battery, which requires two separate electrodes per cell, a bipolar battery is comprised of bipolar electrodes upon which both the anodic and cathodic reactions occur. As with a monopolar battery, the cells in a bipolar battery are electronically connected in series. Unlike a monopolar battery, where the cells are hydraulically isolated, the cells of a flowing electrolyte bipolar battery are hydraulically connected in parallel. Application Ser. No. 189,363 filed May 2, 1988, now abandoned entitled "Terminal Electrode" and having the same assignee as the present application, describes the current flow and structure of a bipolar battery of the zinc-bromine type and is incorporated by way of reference herein for such detail.
Generally, however, flowing electrolyte bipolar batteries require, in addition to cell stack components, electrolyte manifolds and fluid anolyte/catholyte pumps, and end blocks at each end of the battery which sandwich the cell stack therebetween. The end blocks serve as a supporting structure for the cell stack and provide the framework for duct and shunt tunnels to communicate with interiorly disposed elements of the battery. Additionally, the blocks support the various terminal studs which electrically communicate with the end or terminal electrodes of the cell stack. Not only must the blocks necessarily be inert relative to the various chemical constituencies comprising the fluid anolyte/catholyte, it is important that end blocks resist bending or bowing caused primarily by the different pressures which exist between the atmosphere and internal operating environments of the battery. For example, in a zinc-bromine battery environment, operating pressures may reach 15 pounds per square inch. Bowing of end plates may result in nonuniform electrolyte distribution in end cells. Nonuniform flow distribution may then cause a significant reduction in voltage and/or discharge capacity during discharge in, those cells relative to other cells.
Additionally, in zinc-bromine batteries, bowing of end blocks may result in poor zinc plating at the other end cell causing undesirable dendritic zinc growth to occur through the pore structure of the separator of the adjacent end cell. If dendritic growth reaches a cathode surface, it would provide a short circuit for current in that cell and eliminate its voltage contribution.
Various attempts have been made in the prior art to provide end plates which minimize bowing and resulting problems. For example, steel plates coated with an inert plastic material have been employed as end blocks largely at the sacrifice of another important consideration, namely weight. Still other attempts have considered the use of plastic materials ribbed in various patterns to provide additional strength to withstand the internal operating pressures to provide the required rigidity. It is often necessary, however, to have ribs which may approach a thickness of nearly an inch, resulting in an unnecessary increase in the overall volume of the assembly.